Blood cadmium and depressive symptoms: Confounded by cigarette smoking

Our aim was to explore the association between blood cadmium (BCd) and depressive symptoms, adjusting for pack years and blood cotinine, and also stratifying by smoking status. Using data from the US National Health and Nutrition Examination Survey (NHANES) 2005-2012, we categorized depressive symptoms using the PHQ-9 (Patient Health Questionnaire-9) survey and modeled depressive symptoms in relation to BCd adjusted for blood cotinine, pack years of smoking, and other covariates. We also stratified by self-reported smoking status (current, former, never). There were 11,209 subjects from 2005 to 2012, age >= 18 with PHQ-9, smoking, and blood cadmium data available. 876 (7.8%) met criteria for depressive symptoms. Depressive symptoms were associated with BCd levels in a crude model and with adjustment for pack years and cotinine. The association disappeared when analyzed among current, former, or never smokers. Consistent with the literature, we found an association between BCd and depressive symptoms; however, that association disappears in analyses stratified by smoking status. This suggests residual confounding may be present. It is important to stratify by smoking status when investigating health outcomes associated with BCd.